Electrical fuses (eFuses) are used in the semiconductor industry to implement array redundancy, field programmable arrays, analog component trimming circuits, and chip identification circuits. Once programmed, the programmed state of an electrical fuse does not revert to the original state on its own, that is, the programmed state of the fuse is not reversible. For this reason, electrical fuses are called One-Time-Programmable (OTP) memory elements.
The mechanism for programming an electrical fuse is electromigration of a metal semiconductor alloy induced by an applied electrical field and an elevated temperature on a portion of the electrical fuse structure. The metal semiconductor alloy is electromigrated under these conditions from the portion of the electrical fuse structure, thereby increasing the resistance of the electrical fuse structure. The rate and extent of electromigration during programming of an electrical fuse is dependent on the temperature and the current density at the electromigrated portion.
An electrical fuse typically comprises an anode, a cathode, and a fuselink. The fuselink is a narrow strip of a conductive material adjoining the anode and cathode. During programming of the electrical fuse, a positive voltage bias is applied to the anode and a negative voltage bias is applied to the cathode. As electrical current flows through the fuselink having a narrow cross-sectional area, the temperature of the fuselink is elevated. A high current density combined with the elevated temperature at the fuselink facilitates electromigration of the conductive material, which may comprise a metal silicide.
Referring to FIGS. 1A and 1B, a prior art electrical fuse is shown. FIG. 1A is a top-down view of the prior art electrical fuse and FIG. 1B is a vertical cross-sectional view of the prior art electrical fuse in the plane B-B′ in FIG. 1A. The prior art electrical fuse comprises an anode 36, a fuselink 46, and a cathode 56, and is formed on shallow trench isolation 4 located in a semiconductor substrate 2. The anode 36 comprises an anode semiconductor 32 and an anode metal-semiconductor alloy 34. The fuselink 46 comprises a fuselink semiconductor 42 and a fuselink metal-semiconductor alloy 44. The cathode 56 comprises a cathode semiconductor 52 and a cathode metal-semiconductor alloy 54. A gate spacer 55 surrounds the prior art electrical fuse. The anode semiconductor 32, the fuselink semiconductor 42, and the cathode semiconductor 52 comprise a semiconductor material, for example polysilicon. The metal-semiconductor alloys (32, 42, 52) may be formed by metallization of the semiconductor material underneath. If the underlying semiconductor material is polysilicon, the metal-semiconductor alloys are a silicide.
The prior art electrical fuse is programmed by applying a voltage bias between the anode 36 and the cathode 56 to cause a current to flow from the anode 36 to the cathode 56. As the current passes through the fuselink 46, electromigration is induced within the fuselink 46. The current density as well as the temperature of the electromigrated region in the fuselink 46 determines the effectiveness of electromigration. In general, high temperature and high current density in the fuselink 46 are conducive to electromigration. By reducing the width of the electromigrated region in the fuselink 46, the current density and the temperature increases in the electromigrated region during programming of the electrical fuse.
Programming of the prior art electrical fuse typically takes a substantial amount of current, for example, a programming current of about 5 mA for an exemplary prior art electrical fuse having a fuselink width of about 63 nm. A programming transistor capable of supplying such a programming current typically takes up about 3 μm2 of semiconductor area in the case of a silicon based programming transistor. Thus, the programming transistor takes up a substantial fraction of per fuse semiconductor area.
In view of the above, there is a need for an improved electrical fuse that may be programmed by a programming transistor that takes up less semiconductor area.
Particularly, there is a need for an improved electrical fuse that may be programmed with a smaller amount of programming current, and methods of manufacturing the same.